Write-once optical data storage medium having dual recording layers

ABSTRACT

A dual-stack optical data storage medium for write-once recording using a focused radiation beam entering through an entrance face of the medium is described. The medium includes at least one substrate with present on a side thereof a first recording stack L 0  having a write-once type L 0  recording layer with an absorption k L0  and a second recording stack L 1  including a write-once type L 1  recording layer with an absorption k L1 . The first recording stack L 0  has an optical reflection value R L0  and an optical transmission value T L0  and the second recording stack has an optical reflection value R L1 . The first recording stack is present at a position closer to the entrance face than the second recording stacks When the following conditions are fulfilled: 0.45 ≦T L0 ≦0.75 and 0.40 ≦R L1 ≦0.80 and k L0 &lt;0.3 and k L1 &lt;0.3 a dual stack write-once medium is achieved which can be played in a standard DVD-ROM player.

The invention relates to a dual-stack optical data storage medium forwrite-once recording using a focused radiation beam having a wavelengthλ and entering through an entrance face of the medium during recording,comprising:

-   -   at least one substrate with present on a side thereof:    -   a first recording stack named L₀, comprising a write-once type        L₀ recording layer having a complex refractive index        ñ_(L0)=n_(L0)−i.k_(L0) and having a thickness d_(L0), said first        recording stack L0 having an optical reflection value R_(L0) and        an optical transmission value T_(L0),    -   a second recording stack named L₁ comprising a write-once type        L₁ recording layer having a complex refractive index        ñ_(L1)=n_(L1)−i.k_(L1) and having a thickness d_(L1), said        second recording stack L₁ having an optical reflection value        R_(L1), all parameters defined at the wavelength λ,        said first recording stack being present at a position closer to        the entrance face than the second recording stack,    -   a transparent spacer layer sandwiched between the recording        stacks, said transparent spacer layer having a thickness        substantially larger than the depth of focus of the focused        radiation beam.

An embodiment of an optical recording medium as described in the openingparagraph is known from Japanese Patent Application JP-11066622.

Recently the Digital Versatile Disk (DVD) has gained market share as amedium with a much higher data storage capacity than the CD. Presently,this format is available in a read only (ROM), recordable (R) and arewritable (RW) version. For recordable and rewritable DVD, there are atpresent several competing formats: DVD+R, DVD-R for recordable andDVD+RW, DVD-RW, DVD-RAM for rewritable. An issue for both the recordableand rewritable DVD formats is the limited capacity and thereforerecording time because only single-stacked media are present with amaximum capacity of 4.7 GB. Note that for DVD-Video, which is a ROMdisk, dual layer media with 8.5 GB capacity, often referred to as DVD-9,already have a considerable market share. Consequently, recordable andrewritable DVD's with 8.5 GB capacity are highly desired. A dual-layer,i.e. dual-stack, rewritable DVD disk is probably feasible. However, ithas become clear that a rewritable fully compatible disk, i.e. withinthe reflection and modulation specification of the dual-layer DVD-ROM,is very difficult to achieve and requires at least a major breakthroughfor the properties of the amorphous/crystalline phase-change materials,which are used as recording layers in rewritable DVD media. Typicalachievable effective reflection levels are about 7%; such low reflectionvalues severely reduce read-back compatibility on existing DVD-players.Without a full compatibility, the success of a dual-layer rewritable DVDin the market is questionable.

In order to obtain a dual-stack recordable DVD medium which iscompatible with the dual-layer (=dual-stack) DVD-ROM standard, theeffective reflectivity of both the upper L₀ layer and the lower L₁ layershould be at least 18%, i.e. the minimum effective optical reflectionlevel in order to meet the specification is R_(min)=0.18. Effectiveoptical reflection means that the reflection is measured as the portionof effective light coming back from the medium when e.g. both stacks L₀and L₁ are present and focusing on L₀ and L₁ respectively. The minimumreflection R_(min)=0.18 is a requirement of the DVD-standard. However,in practice also a somewhat lower effective reflection, e.g. R>0.12, isacceptable to achieve read-out compatibility in existing DVD-players.Note that such a reflectivity of R>0.12 is, at present, not achievablein a rewritable dual-stack DVD based on e.g. phase-change technology.

The conditions, which must be imposed on the optical reflection,absorption and transmission values of the stacks in order to meet such aspecification are by far not trivial. In JP-11066622 nothing ismentioned about requirements with respect to optical reflection,absorption and transmission values of the stacks and how to achievethese. It should be noted that in this document L₀ is defined as the“closest” stack, i.e. closest to the radiation beam entrance face, andL₁ is the deepest stack, as seen from the radiation beam entrance face.

It is an object of the invention to provide a dual stack optical datastorage medium of the type mentioned in the opening paragraph which haseffective reflection values which provide at least read-outcompatibility with existing DVD-ROM players. In an optimized formcompatibility may be achieved with the existing DVD-ROM standard.

This object is achieved with the optical data storage medium accordingto the invention which is characterized in that 0.45≦T_(L0)≦0.75 and0.40≦R_(L1)≦0.80 and k_(L0)<0.3 and k_(L1)<0.3. The applicant has foundthat these requirements may be deduced from the requirement that theeffective reflection levels from both recording stacks L₀ and L₁ arelarger than 12%. More preferably 0.55≦T_(L0)≦0.65 and 0.50≦R_(L1)<0.70and k_(L0)<0.2 and k_(L1)<0.2 in which case even higher effectivereflection values may be achieved e.g. 15% or 18%. A dual-stackrecordable DVD medium (e.g. DVD+R) based on a recording layer with writeonce technology, having a relatively low optical absorption can inprinciple overcome the reflection-problem of the phase-change rewritableDVD dual stack medium. A write-once recording layer with a relativelylow absorption is e.g. a dye layer. The present invention can be appliedto both the DVD+R and DVD-R formats. In the following, we will use DVD+Rto indicate a recordable DVD in general.

The typical single stack DVD+R medium has a reflectivity of 50% and amodulation of 600%; these values are within the single stack DVD-ROMspecification; DVD+RW media have much lower reflectivity of about 20%.The starting point for developing a dual-stack R-medium is thus muchmore favorable than for RW media. The dye material intrinsically has ahigh transmission at the wavelength λ. In combination with a metalreflective layer, a high reflectivity can be achieved. Thus, recordingis possible with a relatively low absorption in the dye layer. Typicaldyes that can be used are cyanine-type, azo-type, squarylium-type, orother organic dye material having the desired properties.

The minimum effective reflection of both layers is 12%, typical stackdesigns in this invention are targeted for at least R_(eff)=18%. FIG. 1shows that a compatible dual stack DVD+R medium is in principle possiblewhen assuming reasonable numbers for transmission and absorption ofrecording, e.g. dye, stacks. A reflection of larger than 18% per stackis possible if the transmission of the upper recording stack L₀ isbetween 45% and 75%; the intrinsic reflection of the lower recordingstack L₁ should then be in the range 40%-80%. FIG. 1 b illustrates thisfact in more detail.

In the DVD embodiment λ is approximately 655 nm. FIG. 4 a shows thecalculated reflectivity for a recording layer-only stack as a functionof the L₀ recording layer thickness d_(L0). For common values of therecording layer's optical constants (n_(L0)<3, k_(L0)<0.3) asufficiently high transmission can be achieved; the reflectivitytherefore determines the required optical parameters and layerthickness. Preferably n_(L0)≦2.5 and d_(L0) is in the range ofλ/8n_(L0)≦d_(L0)≦3)λ/8n_(L0) or 5λ/8n_(L0)≦d_(L0)≦7λ/8n_(L0). FIG. 4 bshows the calculated maximum reflectivity of a L₀ recording layer-onlystack as a function of the real part of the refractive index n_(L0) ofthe recording layer; the dashed horizontal line indicates a reflectionlevel R=18%. It follows from FIG. 4 b that in order to get areflectivity of at least 18%, the recording layer's refractive indexn_(L0) should be sufficiently large (or small):n_(L0)≧2.5 or n_(L0)≦1.0The latter is however less likely to be met in practice.The optimal L₀ recording layer thickness d_(L0) is at the first orsecond maximum in reflectivity, the preferred layer thickness is then:λ/8n _(L0) ≦d _(L0)≦3λ/8n _(L0)5λ/8n _(L0) ≦d _(L0)≦7λ/8n _(L0)  (2^(nd)max)The advantages of this L₀ stack design are a high transparency and itssimplicity.

In another embodiment a first metal reflective layer, having a thicknessd_(M1)≦25 nm, is present between the write-once L₀ recording layer andthe transparent spacer layer and d_(L0) is in the range ofλ/8n_(L0)≦d_(L0)≦5λ/8n_(L0). For this stack, a relatively thin firstmetal reflective layer is placed between the dye and the spacer. Thefirst metal reflective layer serves as a semi-transparent layer toincrease the reflectivity. A maximum thickness and suitable materialmust be specified to keep the transmission of the first metal reflectivelayer sufficiently high. For the metal layer e.g. Ag, Au, Cu, Al, oralloys thereof, or doped with other elements, can be used. In order toobtain a sufficiently transparent stack (T_(L0)≧45%), the preferredthickness of the metal layer is:d_(M1)≦25 nmThe optimum dye-layer thickness is determined by both the maxima intransmission and reflection.

The presence of the thin metal layer introduces an additional phaseshift Δ˜⅛ to ¼ in the extrema of R and T; for this stack design themaxima in R and T are located at: Max(R)→λ/2n_(L0)(p−Δ),Max(T)→λ/2n_(L0)(p+½−Δ).

Only the thickness range around the first reflection maximum is suitablebecause of the decreasing transmission for larger dye thickness. Thelower limit (LL) for d_(L0) is defined by the maximum in T: LL=Max(R)−½period=λ/8n_(L0). The upper limit for d is defined by 2nd Max(T)−⅛period=Max(R)+⅜ period=5λ/8n_(L0) because for thickness the reflectivitydrops strongly. Thus, the preferred dye layer thickness range becomes:λ/8n _(L0) ≦d _(L0)≦5λ/8n _(L0).

The advantages of the described design are the good reflectiveproperties and the nearly similar stack design, and thus nearly similarfabrication process, as “standard” single recording stack medium.

In another embodiment a first transparent auxiliary layer I1, having arefractive index n_(I1)≧1.8 and having a thickness d_(I1)≦λ2n_(I1), ispresent between the first metal reflective layer and the transparentspacer layer. By adding a first transparent auxiliary layer I1, e.g. adielectric interference layer, adjacent the first metal reflective layerthe transmission near the optimum reflectivity is increased; the role ofthe dielectric I-layer is to counteract the optical mismatch between the“recording layer+thin metal” stack and the substrate, e.g. made ofpolycarbonate, and thereby lower the reflection and raise thetransmission.

Clearly, with three layers many combinations are possible. However, theonly useful stack design is recording layer/thin-metal/I-layer, whichcan have high T, finite R, and sufficient absorption in the recordinglayer at the same time. For this stack type, for the first metalreflective layer e.g. Ag, Au, Cu, Al or alloys thereof, or doped withother elements, can be used. In order to obtain a sufficientlytransparent stack, the preferred thickness of the metal layer for thisstack type is:d_(M1)≦25 nm

As shown in FIGS. 10 a-10 b, the first auxiliary layer I1 below thefirst metal reflective layer indeed increases the stack's transmissionand decreases its reflectivity, while the position of the R- andT-extrema stays (nearly) the same. The optimum recording layer thicknessis determined by the first maximum in reflection, which is given byMax(R)→λ/2n_(L0)(1−Δ), where Δ˜⅛ to ¼ is a phase shift introduced by themetal. The preferred recording layer thickness for this stack becomes:λ/8n _(L0) ≦d _(L0)≦5λ/8n _(L0)

It is preferred that d_(I1)≦λ/4n_(I1). It appears that the relativeincrease in T that can be gained by the first auxiliary I1-layer dependson the I1-layer's refractive index and on the metal reflective layerthickness, while the properties of the recording layer do not influencethe relative increase of T. As shown in FIG. 11, the useful range ofrefractive indices n_(I) of the I-layer isn_(I1)≧1.8

From FIG. 11 it can be derived what the minimal refractive index of theI1-layer is for which an increase by a factor X in transmission of thebare recording layer/thin-metal-reflective layer stack can be gained. Itfollows that for an X-gain in transmission the I-layer's refractiveindex should be:n _(I1)≧(X+0.036*d _(M1)−1.025)/(0.0267*d _(M1)+0.005)

Here, the first metal reflective layer thickness d_(M1) is given innanometers (note that the formula is an approximation in the range ofn_(I1) of 1.8-3). The X-factor can be calculated by dividing therequired transmission of the stack (e.g. 50%) by the transmission of thebare (without I-layer) recording layer/thin-metal-reflective layer stack(e.g. 38%). For example, if a transmission increase by a factor50/38=1.3 is required for the bare recording layer/thin-metal-reflectivelayer stack having a metal reflective layer of 15 nm thickness, therefractive index of the additional I1-layer should by at least 2.0.

The reflection and transmission of the stack are also periodic in thethickness of the lower-lying interference layer, with period λ/2n_(I1).Therefore, the I1-layer thickness need not be larger than one period:d _(I1)≦λ/2n _(I1)

If the I1-layer is intended to increase T (and decrease R) it's optimalthickness lies at the position of the first maximum in T which islocated at (½−Δ)*λ/2n_(I1), with Δ˜⅛.

The preferred (optimal) thickness of the I-layer then becomes:d _(I1,opt)=3λ/16n _(I1)

For larger d_(I1) the transmission decreases and the reflectionincreases again. If n_(I1) is sufficiently large, it is possible to keepthe I1-layer's thickness below the optimum value given above.

The useful thickness range of the I-layer then becomes:d _(I1)≦λ/4n _(I1)The advantage of this design is its flexibility, i.e. a large range of Rand T is possible.

In another embodiment a second transparent auxiliary layer I2, having arefractive index n_(I2) and having a thickness d_(I2) in the range of0<d_(I2)≦3λ/8n_(I2), is present at a side of the write-once L₀ recordinglayer and d_(L0) is in the range of λ/8n_(L0)≦d_(L0)≦3λ/8n_(L0) or5λ/8n_(L0)≦d_(L0)≦7λ/8n_(L0). Preferably the second transparentauxiliary layer is present at a side of the write-once L₀ recordinglayer most remote from the entrance face and n_(I2)≦n_(L0)/1.572.Alternatively the second transparent auxiliary layer is present at aside of the write-once L₀ recording layer closest to the entrance faceand n_(I2)≧n_(L0)/0.636. No metal reflective layer is present. Thisstack is based on the principle of a dielectric mirror. Since the secondauxiliary I2-layer, i.e. the dielectric mirror, is transparent, therequirements for recording- and I-layer thickness and optical constantsfollow from the reflectivity constraint.

The reflectivity is maximized when the interference layer is λ/4n_(I2)(or 3λ/4n_(I2)) thick and the dye layer λ/4n_(L0) (1^(st) max) or3λ/4n_(L0) (2^(nd) max).

The preferred range of the interference layer thickness is:0<d _(I2)≦3λ/8n _(I2)The preferred thickness range for the dye layer is:λ/8n _(L0) ≦d _(L0)≦3λ/8n _(L0)  (1^(st) max)5λ/8n _(L0) ≦d _(L0)≦7λ/8n _(L0)  (2^(nd) max)Two cases of this type of stack can be discerned:

-   (a) recording layer on top of I2-layer and-   (b) I2-layer on top of recording layer.    Case (a)

The optimum reflectivity is given byR=[(1−(n_(L0)/n_(I2))²)/(1+(n_(L0)/n_(I2))²)]².

To meet the reflectivity specification of R=18%, the I2-layer'srefractive index can be calculated to be:n _(I2) ≦n _(L0)/1.572Case (b)

The optimum reflectivity is given byR=[(1−(n_(I2)/n_(L0))²)/(1+(n_(I2)/n_(L0))²)]².

To meet the reflectivity specification of R=18%, the I2-layer'srefractive index can be calculated to be:n _(I2) ≧n _(L0)/0.636By adding more transparent auxiliary layers (with alternating high n andlow n, and thickness around λ/4n) the reflective properties of the stackcan be improved using less extreme values of the refractive indices ofthe I2-layers. However, the stack becomes more complicated then. Theadvantage of the design described above is its relative simplicity whilestill sufficient reflection and high transmission are achieved.

The stacks proposed for L₀ are not restricted to use in dual-stackmedia, but can be used in single-stack and multi-stack (>2) media aswell.

For the L₁ stack of the dual-stack optical data storage medium accordingto the invention a second metal reflective layer is present at a side ofthe write-once type L₁ recording layer most remote from the entranceface. In another embodiment the second metal reflective layer has athickness d_(M2)≧25 nm and preferably d_(L1) is in the range of0<d_(L1)≦3λ/4n_(L1). The latter range is the range of a conventionalsingle stack write once medium. When d_(M2) is lower than 25 nm thereflectivity may become too low. The lower L₁ stack of a recordabledual-stack DVD medium should have high reflectivity at the radiationbeam wavelength in order to be able to read back recorded data throughthe above L₀ stack. To meet the DVD-ROM dual-layer (i.e. dual-stack)specifications the effective reflectivity of L₁ should be in the rangeof 18% to 30%. If the L₀ stack has a transmission at the laserwavelength of T_(L1), this means that the intrinsic reflection of L₁should be in the range 18/T_(L0) ²% to 30/T_(L0) ²%. Given typicaltransmission of L₀ in the range 50% to 60%, this impli L₁'s reflectivityshould be 50% or more. This value already falls within thereflectivity-range specified for single-stack DVD+R discs. Thus inprinciple a single-stack DVD+R stack design can be used as the L₁-stack.However, in the case of type 2 (see FIG. 16 b) discs, this implies thatthe recording layer, e.g. the dye is in direct contact with e.g. theadhesive for the spacer layer. This adhesive can possibly harm the dye,resulting in poor medium lifetimes.

It is therefore preferred that a third transparent auxiliary layer I3,having a refractive index n_(I3) and having a thickness d_(I3) in therange 0<d_(I3)≦λ/n_(I3), is present adjacent the write-once type L₁recording layer at a side of the write-once type L₁ recording layerclosest to the entrance face or that a third metal reflective layer,having a thickness d_(M3) in the range of 0<d_(M3)≦25 nm, is presentadjacent the write-once L₁ recording layer at a side closest to theentrance face and d_(L1) is in the range of 0<d_(L1)≦5λ/16n_(L1) or7λ/16n_(L1)≦d_(L1)≦λ/n_(L1). By introducing the third transparentauxiliary layer or third metal reflective layer the problem of chemicalinfluence of the spacer layer to the recording layer is counteracted.Two L₁ stack types are proposed here which protect the recording layerfrom e.g. the adhesive of the spacer layer. The stacks proposed here arenot restricted to use in dual-stack optical recording media and can beapplied in any (single-stack and multi-stack) organic-recording layer,e.g. dye, based optical recording medium. For the third metal reflectivelayer e.g. Ag, Au, Cu, Al or alloys thereof, or doped with otherelements, can be used.

Schematic layout of this stack design is given in FIG. 14.

For the thin metal layer e.g. Ag, Au, Cu, Al or alloys thereof, or dopedwith other elements, can be used.

In an advantageous embodiment of the dual-stack optical data storagemedium a fourth transparent auxiliary layer I4, having a refractiveindex n_(I4) and having a thickness d_(I4) in the range of0<d_(I4)≦3λ/16n_(I4), is present between the write-once L₁ recordinglayer and the second metal reflective layer. This fourth transparentauxiliary layer allows a slightly thinner recording layer thickness,while the reflection and modulation of written marks remains good.

In yet another advantageous embodiment of the dual-stack optical datastorage medium a fifth transparent auxiliary layer I5, having arefractive index n_(I5) and having a thickness d_(I5) in the range of0<d_(I5)≦λ/4n_(I5), is present adjacent the third metal reflective layerat a side of the third metal reflective layer closest to the entranceface. This fifth transparent auxiliary layer increases the chemicalbarrier between the recording layer and the spacer-layer adhesive thatis beneficial for lifetime of the recording stack.

It may be advantageous when at least one of the transparent auxiliarylayers comprises a transparent heatsink material selected from the groupof materials ITO, HfN and AlON. Generally dielectric materials exhibit apoor heat conductivity. The mentioned materials have a relatively highheat conductivity while they are transparent. A high heat conductivitymay increase the quality of recorded marks in terms of mark definition,e.g. jitter, shape, modulation

In the dual stack optical data storage medium a guide groove for L₁ maybe provided in the transparent spacer layer, called type 1, or in thesubstrate, called type 2. The guide groove is also called pregroove orservo groove. A guide groove for L₀ may be provided in the substrateclosest to the entrance face.

The invention will be elucidated in greater detail with reference to theaccompanying drawings, in which:

FIG. 1 a shows the maximum attainable effective reflection of both theupper recording stack L₀ and the lower recording stack L₁ as a functionof the transmission of upper recording stack L₀;

FIG. 1 b shows the effective total reflection from the L₁ stack as afunction of the intrinsic reflection of L₁; examples for three differentTransmission values of L₀ are shown;

FIG. 2 shows a schematic layout of an embodiment of the optical datastorage medium according to the invention including the two stacks L₀and L₁;

FIG. 3 shows a schematic layout of an embodiment of the L₀ stack of theoptical data storage medium (recording layer-only L₀ stack design);

FIG. 4 a shows the calculated reflectivity as a function of therecording layer thickness d_(L0) for three values of the recording layerrefractive index n_(L0).

FIG. 4 b shows the maximally attainable reflectivity of a singlerecording layer in an optical data storage medium;

FIG. 5 shows a schematic layout of another embodiment of the L₀ stack ofthe optical data storage medium;

FIG. 6 a shows a graph of the transmission of the stack of FIG. 5 as afunction of the recording layer thickness for three values of therefractive index of the recording layer. The dashed line indicates thelower limits allowed for T;

FIG. 6 b shows the reflection of these same stacks as function of therecording layer thickness;

FIG. 7 a shows a schematic layout of another embodiment of the L₀ stackof the optical data storage medium;

FIG. 7 b shows a schematic layout of another embodiment of the L₀ stackof the optical data storage medium;

FIG. 8 a shows the maximum reflectivity of a recording layer/auxiliarylayer stack as a function of the refractive index n_(I) of the auxiliarylayer I for five values of the recording layer's refractive indexn_(L0);

FIG. 8 b shows the maximum reflectivity of an auxiliary layer/recordinglayer stack as a function of the refractive index n_(I) of the auxiliarylayer I for five values of the recording layer's refractive indexn_(L0);

FIG. 9 shows a schematic layout of another embodiment of the L₀ stack ofthe optical data storage medium;

FIG. 10 a shows a comparison between the transmission of a recordinglayer/thin-metal reflective layer stack and a recording layer/thin-metalreflective layer/auxiliary-layer stack as a function of the recordinglayer thickness;

FIG. 10 b shows the same for reflection as a function of the recordinglayer thickness for the stacks of FIG. 10 a.

FIG. 11 shows the maximum factor (X) by which the transmission of arecording layer/thin-metal reflective layer stack can be increased whenadding an auxiliary I-layer adjacent the metal reflective layer;

FIG. 12 shows a schematic layout of an embodiment of the L₁ stack of theoptical data storage medium design: auxiliary layer/recordinglayer/relatively thick metal reflective layer.

FIG. 13 shows the intrinsic reflection of the L₁ stack design of FIG. 12as a function of the recording layer thickness (k_(L1)=0.1) for threevalues of the refractive index n_(L1) of the recording layer. The dashedline indicates the 50% reflection level, which is a practical lowerlimit for the L₁ intrinsic reflectivity.

FIG. 14 shows a schematic layout of another embodiment of the L₁ stackof the optical data storage medium (design: thin metal layer/recordinglayer/relatively thick metal layer).

FIG. 15 shows the intrinsic reflection of the L₁ stack design of FIG. 14as a function of the recording thickness;

FIG. 16 a shows a type 1 optical data storage medium;

FIG. 16 b shows a type 2 optical data storage medium.

In FIG. 1 a the maximum attainable reflection of the lower recordingstack L₁ as a function of the transmission T of upper recording stack L₀is shown. The lines for absorption A=0 are the theoretical limit, atypical absorption for e.g. a dye layer would be 10%. The effectivereflection of 18% per stack as required by the DVD standard is indicatedby the horizontal dashed line; in this invention R_(eff)≧18% is taken asa targeted value for the preferred embodiments.

In FIG. 1 b the effective total reflection from the L₁ stack as afunction of the intrinsic reflection of L₁ is drawn. Examples for threedifferent transmission values T of the L₀ stack are shown;

In FIG. 2 a dual-stack optical data storage medium 10 for recordingusing a focused laser beam 9 having a wavelength 655 nm is shown. Thelaser beam 9 enters through an entrance face 8 of the medium 10 duringrecording. The medium 10 comprises a substrate 7 with present on a sidethereof a first recording stack 5 named L₀, comprising a write-once typeL₀ recording layer 6 having a complex refractive indexñ_(L0)=n_(L0)−i.k_(L0) and having a thickness d_(L0). The firstrecording stack L₀ has an optical reflection value R_(L0) and an opticaltransmission value T_(L0). A second recording stack 2 named L₁comprising a write-once type L₁ recording layer 3 having a complexrefractive index ñ_(L1)=n_(L1)−i.k_(L1) and having a thickness d_(L1) ispresent. The second recording stack L₁ has an optical reflection valueR_(L1). The optical parameters are all measured at the laser beamwavelength. The first recording stack 5 is present at a position closerto the entrance face 8 than the second recording stack 3. A transparentspacer layer 4 is sandwiched between the recording stacks 2 and 5. Thetransparent spacer layer 4 has a thickness substantially larger than thedepth of focus of the focused radiation beam 9.

To meet the DVD-ROM dual layer specification, the effective reflectionlevel from the upper recording stack L₀, being equal to R_(L0), and theeffective reflection level from the lower recording stack L₁, beingequal to R_(L1)*(T_(L0))², should both fall in the range 18% to 30%:0.18≦R_(L0)≦0.30 and 0.18≦R_(L1)*(T_(L0))²≦0.30. In practice, effectivereflection levels>12% are sufficient for read-out compatibility onexisting DVD players. Practical ranges of T_(L0) and R_(L1) for whichthe latter condition can be achieved, are: 0.45≦T_(L0)≦0.75 and0.40≦R_(L1)≦0.80 and k_(L0)<0.3 and k_(L1)<0.3. Thus, with the propercombination of R_(L0), T_(L0) and R_(L1) a DVD+R dual layer (DL) mediumis achieved compatible with the DVD-ROM dual layer specification as faras reflection levels are concerned. A DVD+R DL disc could consist of anycombination of L₀-stack and L₁-stack. One specific embodiment would be:

Medium of type 2 (see FIG. 16 b), with L₀ embodiment stack 5 (95 nmdye/10 nm Ag/55 nm ZnS—SiO₂) and L₁ embodiment stack 3 (15 nm Ag/130 nmdye/100 nm Ag), having transparent spacer 4 thickness of 55 μm.Effective reflection from L₀ is 28%, effective reflection (through L₀)from L₁ is 21%. By using dyes as recording layer, which dyes are almosttransparent at the laser recording wavelength, recording stacks withhigh transmission suitable for multi-stack media can be fabricated. Thisis typically the case in write-once optical media such as CD-R andDVD+R. Below follow different L₀ stack designs, in which an organic dyeis incorporated. The designs have a high transparency (in order toenable addressing lower-lying stacks) and finite reflectivity (necessaryfor read-out). The parameter ranges are tuned such as to meet thespecifications for the upper recording stack L₀ in a recordabledual-stack DVD disc:R_(L0)≧18%,T_(L0)≧50%.The lower limit for T_(L0) may be lower, e.g. 45%, if L₁ is very highlyreflective. To understand the thickness ranges proposed for thedifferent stacks below, it is helpful to note that:

-   (i) The reflection and transmission of the stacks are periodic in    λ/2n.-   (ii) The extrema in reflection and transmission of the stacks nearly    coincide due to the intrinsic high transparency of the recording    layer, e.g. dyes k<n).

In FIG. 3 an embodiment of the L₀ stack of the dual-stack optical datastorage medium 10 is shown having a recording layer 6 wherein n_(L0)242.5 and d_(L0) is in the range of λ/8n_(L0)≦d_(L0)≦3λ/8n_(L0) or5λ/8n_(L0)≦d_(L0)≦7λ/8n_(L0). The symbols have described with FIG. 2.The recording layer 6 is a 59 nm thick azo-dye having a refractive indexñ_(L0)=2.68−i.0.23. The reflection R_(L0)=0.18 and the transmissionT_(L0)=0.58. The wavelength λ is 655 nm.

In FIG. 4 a the calculated reflectivity as a function of the recordinglayer 6 thickness d_(L0) for three values of the recording layer'srefractive index n_(L0) is drawn. Notice that a reflection level of morethan 0.18 can be achieved when the recording layer 6 has a refractiveindex n_(L0) larger than 2.5. The optima in reflection are located atthickness of (½+p)*λ/2with p an integer.

In FIG. 4 b the maximally attainable reflectivity of a single recordinglayer in an optical data storage medium (recording layer is embedded inpolycarbonate background having a refractive index n=1.6) is shown. Thedashed line indicates the value R=18%.

In FIG. 5 an embodiment of the L₀ stack of the dual-stack optical datastorage medium 10 is shown having a recording layer 6 wherein a firstmetal reflective layer 11, having a thickness d_(M1)≦25 nm, is presentbetween the write-once L₀ recording layer 6 and the transparent spacerlayer 4 and d_(L0) is in the range of λ/8n_(L0)≦d_(L0)≦5λ/8n_(L0). Thesymbols have the meaning as described with FIG. 2. The recording layer 6is a 100 nm thick azo-dye (JJAP 37 (1998) 2084) having a refractiveindex ñ_(L0)=2.44−i.0.06. The wavelength λ of th focused laser beam 9 is655 nm.

The following results may be obtained when the first metal reflectivelayer 11 is:

-   -   8 nm Ag (n=0.16, k=5.34) R_(L0)=0.21, T_(L0)=0.53    -   10 nm Au (n=0.28, k=3.9) R_(L0)=0.27, T_(L0)=0.52    -   10 nm Cu (n=0.23, k=3.7) R_(L0)=0.25, T_(L0)=0.55

In FIG. 6 a a calculated graph of the transmission T_(L0) of the stackof FIG. 5 as a function of the recording layer 6 thickness d_(L0) forthree values of the refractive index n_(L0) of the recording layer 6 isshown. The first metal reflective layer 11 is 10 nm Ag. The dashed lineindicates the 50% value for T.

In FIG. 6 b the calculated reflection R_(L0) of this same stack asfunction of the recording layer 6 thickness d_(L0) is shown. The dashedred line indicates the lower limits allowed for R. The maxima in R andminima in T are located at thickness (p−Δ)*λ/2nd, where p is an integerand Δ˜⅛ to ¼. The minima in R and maxima in T are located at thickness(p+½−Δ)*λ/2nd.

In FIGS. 7 a and 7 b two embodiments of the L₀ stack of the dual-stackoptical data storage medium are shown wherein a second transparentauxiliary layer I2 with reference numeral 12, having a refractive indexn_(I2) and having a thickness d_(I2) in the range of0<d_(I2)≦3λ/8n_(I2), is present adjacent the write-once L₀ recordinglayer 6 and d_(L0) is in the range of λ/8n_(L0)≦d_(L0)≦3λ/8n_(L0) or5λ/8n_(L0)≦d_(L0)≦7λ/8n_(L0). In FIG. auxiliary layer 12 is present at aside of the write-once L₀ recording layer 6 most remote from theentrance face 8 and n_(I2)≦n_(L0)/1.572. The second transparentauxiliary layer I2 is a dielectric layer made of SiO₂ with a refractiveindex n_(I2)=1.44 and a thickness d_(I2)=114 nm. The recording layer 6is an azo-dye (JJAP 37 (1998) 2084) having a refractive indexñ_(L0)=2.44−i.0.06 and a thickness d_(L0)=67 nm. The wavelength λ of thefocused laser beam 9 is 655 nm. The following reflection andtransmission may be obtained: R_(L0)=0.20, T_(L0)=0.72.

In FIG. 8 a the maximum reflectivity of a recording layer/auxiliarylayer stack is shown as a function of the refractive index n_(I) of theauxiliary layer I for five values of the recording layer's refractiveindex n_(L0) is shown. The dashed line indicates the 18% value for R.

In FIG. 8 b the maximum reflectivity of an auxiliary layer/recordinglayer stack as a function of the refractive index n_(I) of the auxiliarylayer I for five values of the recording layer's refractive index n_(L0)is shown. The dashed line indicates the 18% value for R.

In FIG. 9 an embodiment of the L₀ stack of the dual-stack optical datastorage medium 10 is shown as in FIG. 5 wherein additionally a firsttransparent auxiliary layer 13 ( I1), having a refractive indexn_(I1)≧1.8 and having a thickness d_(I1)≦λ/2n_(I1), is present betweenthe first metal reflective layer 11 and the transparent spacer layer 4.Preferably d_(I1)≦λ/4n_(I1). The symbols have the meaning as describedwith FIG. 2. The recording layer 6 is a 95 nm thick azo-dye (JJAP 37(1998) 2084) having a refractive index ñ_(L0)=2.44−i.0.06. Thewavelength λ of the focused laser beam 9 is 655 nm.

The following results may be obtained when the first metal reflectivelayer 11 and the first auxiliary layer 13 respectively are:

-   -   10 nm Ag (n=0.16, k=5.34) and 55 nm (ZnS)₈₀(SiO₂)₂₀(n=2.15):        R_(L0)=0.28    -   10 nm Cu (n=0.23, k=3.7) and 20 nm (ZnS)₈₀(SiO₂)₂₀: R_(L0)=0.19,        T_(L0)=

In FIG. 10 a a comparison between the transmission of a recordinglayer/thin-metal reflective layer stack (n_(L0)=2.4, k_(L0)=0.1, 10 nmAg) and a recording layer/thin-metal reflective layer/auxiliary-layerstack (n_(L0)=2.4, k_(L0)=0.1, 10 nm Ag, n_(I)=2.1, d_(I)=50 nm) as afunction of the recording layer thickness d_(L0) is shown. The dashedline indicates the 50% value for T.

In FIG. 10 b the reflection as a function of the recording layerthickness d_(L0) for the stacks of FIG. 10 a is shown. The dashed lineindicates the 18% value for R.

In FIG. 11 the maximum factor (X) by which the transmission of arecording layer/thin-metal reflective layer stack can be increased whenadding an auxiliary I-layer adjacent the metal reflective layer as afunction of the refractive index of the I-layer for three values ofmetal layer thickness. The dashed lines indicate linear approximationsof the functions X(n_(I)) in the range 1.8≦n_(I)≦3.0.

It may be advantageous to apply a transparent heat sink in the L₀recording stack instead of or in addition to the auxiliary layer(s).This may lead to improved recording performance due to heat sink action.The types of L₀ stack that can be used have been described with FIG. 7and FIG. 9. In these two stack types an auxiliary, i.e. dielectriclayer, is present to tune the reflection and transmission values.Typical dielectric materials used are ZnS—SiO2 or SiO₂, etc. Thesedielectric materials have poor heat conductivity, typically <1 W/mK. Animproved heat sink function can be obtained by replacing the dielectricwith for instance ITO, HfN , or AlON which have a heat conductivity >1W/mK (ITO has about 3.6 W/mK). These materials have optical constantsclose to typical dielectrics (n˜2, k<0.05), therefore optical stackdesigns similar to those of e.g. FIG. 7 and FIG. 9 can be used.

In FIG. 12 a schematic layout of an embodiment of the L₁ stack of theoptical data storage medium 10 is shown. The symbols have the meaning asdescribed with FIG. 2. A second metal reflective layer 15 is present ata side of the write-once type L₁ recording layer 3 most remote from theentrance face 8. The second metal reflective layer 15 has a thicknessd_(M1)≧25 nm and

d_(L1) is in the range of 0<d_(L1)≦3λ/4n^(L1). A third transparentauxiliary layer 16 (I3), having a refractive index n_(I3) and having athickness d_(I3) in the range 0<d_(I3)≦λ/n_(I3), is present adjacent thewrite-once type L₁ recording layer 3 at a side of the write-once type L₁recording layer 3 closest to the entrance face 8.

R_(L1) is the intrinsic reflection of the L₁ stack. The effectivereflection as defined in annex D of the DVD read-only-disk book shouldbe in the range 18% ≦R_(L1eff)≦30%

T_(L0) is the intrinsic transmission of the L₀ stack, i.e. for the lowerlying L₁ stack having intrinsic reflection R_(L1) the effectivereflection in a true dual-stack medium will be T_(L0) ²*R_(L1)

The recording layer 3 is a 130 nm thick azo-dye (mat sc. and eng. B79(2001) 45.) having a refractive index ñ_(L0)=2.44-i.0.06. The wavelengthλ of the focused laser beam 9 is 655 nm. The fourth transparentauxiliary layer 16 is 50 nm SiO₂ (n=1.44) and the second metalreflective layer is 100 nm Ag. A reflection R_(L1)=0.73 is achieved.

FIG. 13 shows the intrinsic reflection of the L₁ stack design of FIG. 12as a function of the recording layer thickness (k_(L1)=0.1) for threevalues of the refractive index n_(L1) of the recording layer. The dashedline indicates the 50% reflection level, which is a practical value forthe L₁ intrinsic reflectivity.

In FIG. 14 a schematic layout of an embodiment of the L₁ stack of theoptical data storage medium 10 is shown. The symbols have the meaning asdescribed with FIG. 2. A third metal reflective layer 17, having athickness d_(M3) in the range of 0<d_(M3)≦25 nm, is present at a side ofthe write-once L₁ recording layer 3 closest to the entrance face 8 andd_(L1) is in the range of 0<d_(L1)≦5λ/16n_(L1) or7λ/16n_(L1)≦d_(L1)≦λ/n_(L1).

The second metal reflective layer 15 has a thickness d_(M1)≧5 nm. R_(L1)is the intrinsic reflection of the L₁ stack. The effective reflection asdefined in annex D of the DVD read-only-disk book should be in the range18% ≦R_(L1eff)≦30%. T_(L0) is the intrinsic transmission of the L₀stack, i.e. for the lower lying L₁ stack having intrinsic reflectionR_(L1) the effective reflection in a true dual-stack medium will beT_(L0) ²*R_(L1)

The recording layer 3 is a 150 nm thick azo-dye (JJAP 37 (1998) 2084.)having a refractive index ñ_(L0)=2.44−i.0.06. The wavelength λ of thefocused laser beam 9 is 655 nm. The third metal reflective layer 17 is15 nm Ag. A reflection R_(L1)=0.8 is achieved.

In another embodiment (not drawn) a fourth transparent auxiliary layerI4, having a refractive index n_(I4) and having a thickness d_(I4) inthe range of 0<d_(I4)≦3λ/16n_(I4), may be present between the write-onceL₁ recording layer and the second metal reflective layer 15. In thelatter case the recording layer 3 is a 55 nm thick azo-dye (JJAP 37(1998) 2084.) having a refractive index ñ_(L0)=2.44−i.0.06. Thewavelength λ of the focused laser beam 9 is 655 nm. The third metalreflective layer 17 is 10 nm Au. The fourth transparent auxiliary layeris 60 nm SiO₂. A reflection R_(L1)=0.63 is achieved.

In FIG. 15 the intrinsic reflection of the L₁ stack design of FIG. 14 asa function of the recording thickness (k_(L1)=0.1) for three values ofthe refractive index n_(L1) of the recording layer is shown. The dashedline indicates the 50% reflection level, which is a practical lowerlimit for the L₁ intrinsic reflectivity.

In FIG. 16 a a so-called type 1 medium is described. An opticalrecording stack (L₀), optically semi-transparent at the laserwavelength, is applied to a transparent, pre-grooved substrate 7. Atransparent spacer layer 4 is attached to the L₀ stack. The spacer layer4 either contains pregrooves (G) for L₁ or pregrooves (G) for L₁ aremastered into the spacer layer 4 after application to L₀. Secondrecording stack L₁ is deposited on the grooved spacer layer 4. Finally,a counter substrate 1 is applied.

In FIG. 16 b a so-called type 2 medium is described. An opticalrecording stack (L₀), optically semi-transparent at the laserwavelength, is applied to a transparent, pre-grooved substrate 7. Asecond optical recording stack L₁, reflective at the laser wavelength,is applied to a second transparent pre-grooved (G) substrate 1. Thissubstrate 1 with L₁ is attached to the substrate 7 with L₀ with atransparent spacer layer 4 in between. Preferred spacer-layer thicknessfor both disc types is 40 μm to 70 μm.

The stacks proposed in this document are not restricted to use inDVD+R-DL and can be applied in any (multi-stack) organic-dye basedoptical recording medium. The thickness and optical constant rangesspecified, however, are such as to meet the requirements for an L₀- andL₁-stack of a DVD+R-DL medium. It should be noted that the actualrecording of marks does not necessarily take place in the groove G butmay take place in the area between grooves, also referred to as on-land.In this case the guide groove G merely serves as a servo tracking meanswith the actual radiation beam recording spot being present on-land.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The mere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage.

According to the invention a dual-stack optical data storage medium forwrite-once recording using a focused radiation beam entering through anentrance face of the medium is described. The medium comprises at leastone substrate with present on a side thereof a first recording stacknamed L₀, comprising a write-once type L₀ recording layer with anabsorption k_(L0) and a second recording stack named L₁ comprising awrite-once type L₁ recording layer with an absorption k_(L1). The firstrecording stack L₀ has an optical reflection value R_(L0) and an opticaltransmission value T_(L0) and the second recording stack has an opticalreflection value R_(L1). The first recording stack is present at aposition closer to the entrance face than the second recording stack.When the following conditions are fulfilled: 0.45≦T_(L0)≦0.75 and0.40≦R_(L1)≦0.80 and k_(L0)<0.3 and k_(L1)<0.3 a dual stack write-oncemedium is achieved which can be played in a standard DVD-ROM player.Several stack designs are described fulfilling the above conditions.

1. A dual-stack optical data storage medium for write-once recordingusing a focused radiation beam having a wavelength λ and enteringthrough an entrance face of the medium during recording, comprising: atleast one substrate with present on a side thereof: a first recordingstack L₀ comprising a write-once type first recording layer having acomplex refractive index ñ_(L0)=n_(L0)−i.k_(L0) and having a thicknessd_(L0), said first recording stack L₀ having an optical reflection valueR_(L0) and an optical transmission value T_(L0), a second recordingstack L₁ comprising a write-once type second recording layer having acomplex refractive index ñ_(L1)=n_(L1)−i.k_(L1) and having a thicknessd_(L1), said second recording stack L₁ having an optical reflectionvalue R_(L1), all parameters being defined at the wavelength λ, saidfirst recording stack being present at a position closer to the entranceface than the second recording stack, a transparent spacer layersandwiched between the recording stacks, said transparent spacer layerhaving a thickness substantially larger than the depth of focus of thefocused radiation beam, wherein 0.45≦T_(L0)≦0.75 and 0.40≦R_(L1)≦0.80and k_(L0)<0.3 and k_(L1)<0.3.
 2. A dual-stack optical data storagemedium as claimed in claim 1, wherein λ is approximately 655 nm.
 3. Thedual-stack optical data storage medium as claimed in claim 1, whereinfor the write-once first recording layer the following conditions arefulfilled n_(L0)≧2.5 and d_(L0) is in the range ofλ/8n_(L0)≦d_(L0)≦3λ/8n_(L0) or 5λ/8n_(L0)≦d_(L0)≦7 λ/8n_(L0).
 4. Thedual-stack optical data storage medium as claimed in claim 1, wherein afirst metal reflective layer, having a thickness d_(M1)≦25 nm, ispresent between the write-once first recording layer and the transparentspacer layer and d_(L0) is in the range of λ/8n_(L0)≦d_(L0)≦5λ/8n_(L0).5. The dual-stack optical data storage medium as claimed in claim 4,wherein a transparent auxiliary layer I1, having a refractive indexn_(I1)≧1.8 and having a thickness d_(I1)≦λ/2n_(I1), is present betweenthe first metal reflective layer and the transparent spacer layer. 6.The dual-stack optical data storage medium as claimed in claim 5,wherein d_(I1)≦λ/4n_(I1).
 7. The dual-stack optical data storage mediumas claimed in claim 1, wherein a transparent auxiliary layer I2, havinga refractive index n_(I2) and having a thickness d_(I2) in the range of0<d_(I2)≦3λ/8n_(I2), is present at a side of the write-once firstrecording layer and d_(L0) is in the range ofλ/8n_(L0)≦d_(L0)≦3λ/8n_(L0) or 5λ/8n_(L0)≦d_(L0)≦7λ/8n_(L0).
 8. Thedual-stack optical data storage medium as claimed in claim 7, whereinthe transparent auxiliary layer is present at a side of the write-oncefirst recording layer most remote from the entrance face andn_(I2)≦n_(L0)/1.572.
 9. The dual-stack optical data storage medium asclaimed in claim 7, wherein the transparent auxiliary layer is presentat a side of the write-once first recording layer closest to theentrance face and n_(I2)≧n_(L0)/0.636.
 10. The dual-stack optical datastorage medium as claimed in claim 4, wherein a second metal reflectivelayer is present at a side of the second recording stack L₁ recordinglayer most remote from the entrance face.
 11. The dual-stack opticaldata storage medium as claimed in claim 10, wherein the second metalreflective layer has a thickness d_(M1)≧25 nm.
 12. The dual-stackoptical data storage medium as claimed in claim 11, wherein d_(L1) is inthe range of 0<d_(L1)≦3λ/4n_(L1).
 13. The dual-stack optical datastorage medium as claimed in claim 12, wherein a transparent auxiliarylayer I3, having a refractive index n_(I3) and having a thickness d_(I3)in the range 0<d_(I3)≦λ/n_(I3), is present adjacent the write-once typesecond recording layer at a side of the write-once type second recordinglayer closest to the entrance face.
 14. The dual-stack optical datastorage medium as claimed in claim 11, wherein a third metal reflectivelayer, having a thickness d_(M3) in the range of 0<d_(M3)≦25 nm, ispresent at a side of the write-once first recording layer closest to theentrance face and d_(L1) is in the range of 0<d_(L1)≦5λ/16n_(L1) or7λ/16n_(L1)≦d_(L1)≦λ/n_(L1).
 15. The dual-stack optical data storagemedium as claimed in claim 12, wherein a transparent auxiliary layer I4,having a refractive index n_(I4) and having a thickness d_(I4) in therange of 0<d_(I4)≦3λ/16n_(I4), is present between the write-once firstrecording layer and the second metal reflective layer.
 16. Thedual-stack optical data storage medium as claimed in claim 13, wherein afurther transparent auxiliary layer I4, having a refractive index n_(I4)and having a thickness d_(I4) in the range of 0<d_(I4)≦3λ/16n_(I4), ispresent between the write-once first recording layer and the secondmetal reflective layer.
 17. The dual-stack optical data storage mediumas claimed in claim 14, wherein a further transparent auxiliary layerI5, having a refractive index n_(I5) and having a thickness d_(I5) inthe range of 0<d_(I5)≦3λ/16n_(I5), is present adjacent the third metalreflective layer at a side of the third metal reflective layer closestto the entrance face.
 18. The dual-stack optical data storage medium asclaimed in claim 5, wherein at least one of the transparent auxiliarylayer comprises a transparent heatsink material selected from the groupof materials ITO, HfN and AlON.
 19. The dual-stack optical data storagemedium as claimed in claim 1, wherein a guide groove for write-once typesecond recording layer is provided in the transparent spacer layer. 20.The dual stack optical data storage medium as claimed in claim 1,wherein a guide groove (G) for write-once type first recording layer isprovided in the substrate.